Selectively configurable circuit board

ABSTRACT

Embodiments of the invention provide thermally actuatable switches and selectively configurable circuit boards which may employ such switches. A circuit board of one embodiment includes a substrate having board leads and a plurality of electrical connectors arranged adjacent a component site. Selectively configurable circuitry may be carried by the substrate and adapted to selectively couple selected ones of the electrical connectors to selected ones of the board leads. One or more trace may be associated with each of the electrical connectors and one or more of these traces may include a thermally actuatable switch that can be selectively closed. The thermally actuatable switch may comprise a gap between two conductive lengths of the conductive trace, an exposed switch surface, and a thermally responsive member that may wet the exposed switch surface when selectively heated above an activation temperature.

TECHNICAL FIELD

The present invention provides certain improvements in microelectronicdevice assemblies. The invention has particular utility in connectionwith configuring circuit boards for use with different microelectroniccomponents or different configurations of microelectronic components

BACKGROUND

The microelectronic device industry is highly competitive. To maintain acompetitive edge, manufacturers must be able to quickly adapt theirproduct lines to advancing technology and changing consumer demands.Many microelectronic products require a number of separate components,one or more of which must be dedicated to a particular product design.If a manufacturer orders an inventory of microelectronic componentsdedicated to one particular product, the inventory may have to bediscarded or sold well below cost if sales of the product fall short ofprojected levels.

The memory module industry illustrates the difficulties inherent inpredicting the market and minimizing manufacturing costs. Many computersand other processor-based systems employ either Single In-line MemoryModules (SIMMs) or Dual In-line Memory Modules (DIMMs). SIMMs and DIMMseach generally comprise a circuit board with a plurality of integratedcircuit dies mounted thereon. The dies are often interchangeable and canbe used on a wide variety of different SIMM or DIMM configurations. Thecircuit boards, however, are commonly specific to a particular SIMM orDIMM configuration. Manufacturer will order or produce an inventory ofcircuit boards for a particular SIMM or DIMM configuration. If marketdemands for that particular configuration fall short of projecteddemands, the manufacturer will be unable to use the inventory ofspecialized circuit boards for another memory module product.Oftentimes, if the manufacturer overestimates the demand for aparticular memory module configuration, the demand for an alternativeconfiguration will be underestimated. It can sometimes take weeks toredesign and stock an alternative configuration, leading to productiondelays and backlogs in customer orders.

U.S. Pat. No. 5,377,124 (Mohsen, the teachings of which are incorporatedherein by reference) suggests a field programmable printed circuit boardwhich employs a relatively complex, multi-layered circuit board and aspecialized integrated circuit die, or “programmable interconnect chip,”mounted on the circuit board. The programmable interconnect chipincludes circuitry which will route connections between the conductivetraces provided on the rest of the circuit board. Ostensibly, byreplacing one programmable interconnect chip with a differentprogrammable interconnect chip, the circuit board can be adapted fordifferent uses. Unfortunately, designing and producing such specializedintegrated circuit dies can be a relatively expensive, time-consumingprocess. With some lower profit margin products, e.g., standard memorymodules, the cost of such a specialized die may well outweigh thepotential cost savings afforded by the adaptability of the basic circuitboard.

Manufacturers of memory modules and other microelectronic deviceassemblies commonly test each module before it is shipped. If one of theintegrated circuit dies mounted on the module is defective, the entiremodule may need to be discarded. In U.S. Pat. No. 5,953,216, theteachings of which are incorporated herein by reference, Farnworth etal. propose an apparatus and method for substituting a replacementdevice (e.g., a new integrated circuit die) for a defective component(e.g., a defective integrated circuit die). In accordance with thismethod, the defective component may be isolated by severing electricalconnections between the component and the circuit board or the like towhich the defective component is mounted. The replacement component maybe attached to a replacement site on the circuit board and coupled to adedicated replacement terminal on the circuit board, e.g., bywirebonding. Farnworth et al. employ a circuit board that includes areplacement site for all of the modules produced, including the majorityof the modules that do not include any defective components. Leaving anempty replacement site in defect-free modules may be undesirable in somecircumstances.

SUMMARY

Embodiments of the invention provide actuatable traces formicroelectronic assemblies, selectively configurable circuit boards,processor-based devices employing selectively configurable circuitboards, and methods of selectively configuring a circuit board. Anactuatable trace for a microelectronic assembly in one embodimentincludes a first conductive length and a second conductive length. A gapis disposed between, and electrically separates, the first and secondconductive lengths and has an exposed gap surface. A fusible member isin communication with the gap. The fusible member is spaced from thefirst and second lengths and is formed of a fusible material which, whenmelted, will wet the gap surface to electrically connect the first andsecond conductive lengths across the gap. If so desired, the firstconductive length may be formed of a first conductive material and thesecond conductive length may be formed of a second conductive material,with the fusible material having a melting point below the melting pointof the first conductive material and the melting point of the secondconductive material.

Another embodiment of the invention provides a selectively configurablecircuit board. The circuit board may include a substrate and circuitrycarried by the substrate. The substrate may include at least onecomponent site for receiving a microelectronic component. The circuitrymay include a plurality of selectively actuatable traces associated withthe component site. At least one of the actuatable traces may comprisean actuatable trace in accordance with the previously describedembodiment. In one adaptation of such a circuit board, a second one ofthe actuatable traces can comprise a third conductive length formed of athird conductive material, a fourth conductive length formed of a fourthconductive material, and a gap between the third and fourth conductivelengths. A fused bridge may span the gap to electrically connect thethird and fourth conductive lengths. The fused bridge may be formed of aconductive material which is different from, and has a lower meltingpoint than, the third conductive material and the fourth conductivematerial.

An alternative embodiment of the invention provides a selectivelyconfigurable circuit board that includes a substrate having at least onecomponent site adapted to receive a microelectronic component. Aplurality of board leads may be adapted to interface the circuit boardwith an external bus. A plurality of electrical connectors may bearranged adjacent the component site, with the electrical connectorsbeing adapted to be electrically coupled to a contact of amicroelectronic component which may be received at the component site.Selectively configurable circuitry may be carried by the substrate andadapted to selectively couple selected ones of the electrical connectorsto selected ones of the board leads. The selectively configurablecircuitry may comprise at least one trace associated with each of theelectrical connectors, with at least one of the traces including anormally open thermally actuatable switch that can be selectively closedto create an electrical connection. The thermally actuatable switch maycomprise a gap between two conductive lengths of the conductive trace,an exposed switch surface, and a thermally responsive member that maywet the exposed switch surface when selectively heated above anactivation temperature.

A further embodiment of the invention provides a programmable computerthat includes a system bus, a processor coupled to the system bus, and aselectively configured circuit board. The circuit board may comprise asubstrate having at least one component site and a microelectroniccomponent carried by the substrate at the component site, with themicroelectronic component including a plurality of contacts. A pluralityof board leads may be coupled to the system bus. A plurality ofelectrical connectors may be arranged adjacent the component site, withat least some of the electrical connectors being individually coupled tothe microelectronic component contacts. A first trace may be carried bythe substrate and electrically connect one of the electrical connectorsto one of the board leads. The first trace may include a closed switchthat comprises a normally open thermally actuatable switch that has beenclosed to create an electrical connection. A second trace may also becarried by the substrate and coupled to one of the electrical connectorsand one of the board leads. The second trace may include an openthermally actuatable switch that can be selectively closed to create anelectrical connection.

Still another embodiment of the invention provides a method ofmanufacturing a microelectronic device assembly including amicroelectronic component and a circuit board. Each of a plurality ofcomponent contacts of the microelectronic component may be electricallycoupled to one of a plurality of board contacts carried by the circuitboard. The circuit board may carry a plurality of configurable tracesassociated with the board contacts and each of the configurable tracesmay include at least one normally open thermally actuatable switch. Afirst normally open actuatable switch is identified from the pluralityof normally opened thermally actuatable switches. The first switch maybe locally heated to selectively close the first switch to define anelectrical pathway between at least one of the board contacts and atleast one of a plurality of board leads the carried by the circuitboard.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a selectively configurable circuitboard in accordance with one embodiment of the invention.

FIG. 2 is a schematic illustration of the selectively configurablecircuit board of FIG. 1 in a first configuration.

FIG. 3 is a schematic illustration of the selectively configurablecircuit board of FIG. 1 in a second configuration.

FIGS. 4A-E schematically illustrate stages in the manufacture of athermally actuatable switch in accordance with one embodiment of theinvention.

FIG. 4F is a top elevation view of the thermally actuatable switch ofFIG. 4E.

FIG. 5 is a schematic illustration of the thermally actuatable switch ofFIG. 4E after it has been thermally actuated to create an electricalconnection.

FIG. 6 is a schematic cross-sectional illustration of a thermallyactuatable switch in accordance with an alternative embodiment of theinvention.

FIG. 7 is a schematic cross-sectional illustration of the thermallyactuatable switch of FIG. 6 after it has been closed to create anelectrical connection.

FIGS. 8A-B schematically illustrate stages in the manufacture of athermally actuatable switch in accordance with another embodiment of theinvention.

FIG. 9 is a schematic illustration of the thermally actuatable switch ofFIG. 8B after it has been thermally actuated to create an electricalconnection.

FIG. 10 is a schematic illustration of a processor-based system inaccordance with a further embodiment of the invention.

DETAILED DESCRIPTION

Various embodiments of the present invention provide selectivelyconfigurable circuit boards, actuatable traces for microelectronicassemblies, processor-based devices employing such circuit boards, andmethods of selectively configuring a circuit board. The followingdescription provides specific details of certain embodiments of theinvention illustrated in the drawings to provide a thoroughunderstanding of those embodiments. It should be recognized, however,that the present invention can be reflected in additional embodimentsand the invention may be practiced without some of the details in thefollowing description.

Selectively Configurable Circuit Boards

FIG. 1 is a schematic illustration of a selectively configurable circuitboard 10 in accordance with one embodiment of the invention. Thisembodiment includes a substrate 20 carrying circuitry 30 adapted toselectively interconnect selected board contacts 32 a-p with selectedboard leads L₁₋₁₆. The substrate 20 may be flexible or rigid and haveany desired configuration. The substrate 20 may be formed of materialscommonly used in microelectronic substrates, such as ceramic, silicon,glass, or combinations thereof. The substrate 20 can alternatively beformed of an organic material or the like commonly employed for printedcircuit boards (PCBs). In one embodiment of the invention, the substrate20 comprises a printed circuit board such as an FR-4 PCB. The size andshape of the substrate 20 can be varied as desired. For example, thesubstrate 20 may conform to industry standard specifications for a SIMMor DIMM.

The substrate 20 also includes one or more component sites 25, each ofwhich may be adapted to receive a microelectronic component 70. In theillustrated embodiment, the substrate 20 includes a first component site25 a and a second component site 25 b. The first component site 25 a maybe adapted to receive a first microelectronic component 70 a and thesecond component site 25 b may be adapted to receive a secondmicroelectronic component 70 b.

A plurality of board contacts 32 may be arranged adjacent to each of thecomponent sites. In the particular embodiment shown in FIG. 10, whichmay comprise a standard 16-pin DIMM, each of the component sites 25 a-bis associated with eight board contacts 32. Hence, the first componentsite 25 a is associated with a first set of eight board contacts 32 a-hand the second component site 25 b is associated with a second set ofboard contacts 32 i-p. Each of the board contacts 32 is adapted to beelectrically coupled to a component contact 72 of one of themicroelectronic components 70. Hence, the first set of board contacts 32a-h may be adapted for electrical coupling to the component contacts 72a-h, respectively, of the first microelectronic component 70 a.Similarly, the board contacts 32 i-p of the second component site 25 bmay be adapted for electrical coupling to the component contacts 72 i-p,respectively, of the second microelectronic component 70 b. The boardcontacts 32-a-p may take any desired form. For example, the boardcontacts 32 a-p may comprise holes and the component contacts 72 a-p maycomprise pins received in the holes. Alternatively, the board contacts32 a-p may comprise bond pads of the type commonly used in wirebondingor flip chip bonding. The board contacts 32 shown in FIG. 1 are arrangedalong a single edge of their respective component sites 25. It should berecognized that this is merely for purposes of illustration and theboard contacts 32 may be arranged in an array within the component sites25 (e.g., where flip chip bonding is used), around the periphery of thecomponent sites 25 (e.g., where wirebonding is to be used), or in anyother suitable arrangement.

As noted above, the circuit board 10 of FIG. 1 may be used as thecircuit board of a 16-pin DIMM. The circuit board 10 includes sixteenboard leads L₁₋₁₆, which may be arranged in two sets of eight leads(L₁₋₈ and L₉₋₁₆). In conventional circuit boards (not shown), anindividual board contact 32 would be directly wired to a specific one ofthe board leads L by a single conductor carried by the substrate. In theembodiment of FIG. 1, however, none of the board contacts 32 areelectrically connected to any of the board leads L. Instead, thecircuitry 30 which can be used to connect the board contacts 32 to theleads L includes a series of short circuit traces which may be actuatedto selectively connect specific board contacts 32 to specific desiredleads L as the situation demands.

Each of the board contacts 32 a-p may be coupled to a separate contacttrace segment 34 a-p, respectively. Similarly, each of the leads L₁₋₁₆may be coupled to a lead trace segment 36 a-p, respectively. In theinitial state shown in FIG. 1, none of the contact trace segments 34 areelectrically connected to any of the lead trace segments 36. Instead, anormally open thermally actuatable first switch 50 is disposed betweeneach of the contact trace segments 34 and at least one of the lead tracesegments 36. In the particular embodiment shown in FIGS. 1-3, sixteenfirst switches 50 a-p are employed, with each of the first switches 50a-p being disposed between and electrically separating one of thecontact trace segments 34 a-p from one of the lead trace segments 34a-p, respectively. Hence, the open first switch 50 a is disposed betweenthe contact trace segment 34 a and the lead trace segment 36 a andserves to divide the trace between the connector 32 a to the lead L intothe two electrically separate trace segments.

The illustrated circuit board 10 further includes a set of normally openthermally actuatable second switches 52 a-p. Each board contact 32 andeach lead L is associated with one of the first switches 50 and with oneof the second switches 52. As shown in FIG. 1, a plurality ofalternative contact trace segments 40 a-p may provide a conductive pathfrom one of the second switches 52 a-p, respectively, to one of theboard contacts 32. A first set of the second switches 52 a-h isassociated with the second set of board contacts 32 i-p, respectively,whereas a second set of the second switches 52 i-p are associated withthe first set of board contacts 32 a-h, respectively. A series ofalternative lead trace segments 42 a-p may provide a conductive pathfrom each of the board leads L₁₋₁₆ to one of the second switches 52 a-p,respectively.

FIG. 2 schematically illustrates the circuit board 10 of FIG. 1 with themicroelectronic components 70 a-b received at one of the component sites25 a-b (shown in FIG. 1, but not visible in FIG. 2). Although not shownin FIG. 2, each of the component contacts 72 of the microelectroniccomponents 70 may be electrically coupled to one of the board contacts32 of the board circuitry 30.

Each of the first switches 50 in the microelectronic device assembly ofFIG. 2 has been selectively closed, while each of the second switches 52remains in its normally open state. As a result, each of the contacttrace segments 34 a-p is electrically connected to one of the lead tracesegments 36 a-p, respectively, by one of the first switches 50 a-p,respectively. This defines a plurality of traces which couple the firstmicroelectronic component 70 a to the first set of board leads L₁₋₈ andcouple the second microelectronic component 70 b to the second set ofboard leads L₉₋₁₆. This is in keeping with the conventional connectionof a pair of integrated circuit dies to the leads on an edge connectorin a conventional DIMM.

In the configuration shown in FIG. 3, the microelectronic components 70are mounted to the substrate 20, as in FIG. 2. However, each of thefirst switches 50 remains in its open state in FIG. 3. The first set ofsecond switches 52 a-h has been selectively closed, but the remainder ofthe second switches 52 remains open. As a result, the alternative leadtraces 42 a-h are electrically connected to the alternative contacttraces 40 a-h. This, in turn, serves to electrically couple thecomponent contacts 72 i-p (FIG. 1) of the second microelectroniccomponent 70 b to the first set of board leads L₁₋₈. Because theremainder of the second switches 52 and all of the first switches 50remain in their normally open state, the first microelectronic component70 a remains electrically isolated, i.e., it is not electrically coupledto any of the board leads L.

The configuration shown in FIG. 3 may be advantageous if the firstmicroelectronic component 70 a is defective. For example, inmanufacturing a 16-pin DIMM, the first integrated circuit die orintegrated circuit package 70 a may be determined to be defective. If aconventional circuit board were employed, the entire DIMM would have tobe scrapped. The second integrated circuit die 70 b may be entirelyfunctional, but it is connected to the second set of leads L₉₋₁₆,because the computer will first address the first set of leads L₁₋₈,which are connected to the defective die 70 a, the module would notoperate properly.

Rather than scrapping every DIMM having a defective die 70 a, theconfiguration of FIG. 3 enables the manufacturer to connect the firsteight pins (leads L₁₋₈) to the second die 70 b. With the defective die70 a isolated on the circuit board 10, the product shown in FIG. 3 maybe utilized as a conventional 8-pin memory module. While the 8-pinmodule may sell for less than the intended 16-pin product, this is stillan appreciable improvement over scrapping the entire DIMM because of asingle defective integrated circuit die.

FIGS. 1-3 illustrate one particular embodiment of the invention whichmay be useful in manufacturing DIMMs. One of ordinary skill in the artwill recognize, though, that the flexibility afforded by the selectivelyconfigurable circuitry 30 need not be limited to the particularapplication shown. FIGS. 1-3 illustrate a relatively simple embodimentwherein each of a series of electrical connectors 32 may be selectivelyconnected to each of two different electrical leads L. It should bereadily apparently to one skilled in the art that the principles of theinvention need not be so limited, however, and the possible circuitrycan be increased significantly by increasing the number of switches andalternative traces associated with any particular electrical contact orlead.

Thermally Actuatable Switches

As noted above, embodiments of the invention employ thermally actuatableswitches. In the embodiment of FIGS. 1-3, these thermally actuatableswitches permit the circuitry 30 of the circuit board 10 to beselectably configured as desired to connect board contacts 32 to boardleads L in a desired arrangement. These thermally actuatable switchesare normally open, i.e., do not provide a conductive path thereacross.By thermally actuating a particular switch, however, the switch can beselectively closed to define a conductive path across the switch. Thiscan be used to connect selected contact trace segments 34 to selectedlead trace segments 36, for example. Each of the switches may be adaptedto be individually closed in response to a localized thermal stimuluswithout necessitating closure of any other switch. This provides a greatdeal of flexibility in configuring the circuitry 30.

FIGS. 4A-F illustrate a thermally actuatable switch 50 in accordancewith one embodiment of the invention. FIGS. 4A-F illustrate one of thefirst switches 50 of FIGS. 1-3, but this should not be deemed as overlylimiting. If so desired, the second switches 52 shown in FIGS. 1-3 mayhave the same design and be formed in the same manner shown in FIGS.4A-F. Additionally, the various thermally actuatable switches shown inFIGS. 4-9 can be utilized in a variety of different applications andneed not be limited to the particular design shown in FIGS. 1-3 ordiscussed above.

FIG. 4A shows an initial stage in the manufacture of a thermallyactuatable switch 50. This initial stage includes a substrate 20 with afirst conductive trace segment 34 and a second conductive trace segment36 carried on an exterior surface of the substrate 20. These tracesegments 34 and 36 are spaced from one another by a non-conductive gap80. A portion of the exterior surface 22 of the substrate 20 defines anexposed gap surface or switch surface 82 between the trace segments 34and 36. For reasons explained below, it may be desirable to provide awetable coating 83 on the gap surface 82 to enhance performance of theswitch 50.

Using conventional photoresist techniques, a resist layer 85 may beapplied over the exterior surface 22 of the substrate 20 and theconductive trace segments 34 and 36 (FIG. 4B). This photoresist layercan be treated and selectively stripped to expose a portion of the gapsurface 82 (FIG. 4C) employing known processes. A portion of thephotoresist desirably remains within the gap 80. In particular, a firstlateral thickness 85 a of the resist layer is disposed between the firsttrace segment 34 and the gap 80. Similarly, a second lateral thickness85 b of the resist layer is disposed between the second trace segment 36and the gap 80.

As shown in FIG. 4D, a thermally responsive member 90 may be depositedin the gap 80 to span the distance between the two lateral thicknesses85 a-b of the resist layer 85. Thereafter, the remaining resist layer 85may be stripped, leaving a first gap portion 80 a between the thermallyresponsive member 90 and the first conductive trace segment 34 and asecond gap portion 80 b between the thermally responsive member 90 andthe second conductive trace segment 36. As shown in FIG. 4E-F, a firstgap surface portion 82 a is exposed by the first gap portion 80 a and asecond gap surface portion 82 b is exposed by the second gap portion 80b.

The first gap segment 80 a may have a width W_(a) approximately equal tothe first lateral thickness 85 a of the resist layer 85 (FIG. 4D) andthe second gap portion 80 b may have a width W_(b) approximately equalto the second lateral thickness 85 b. The widths W_(a) and W_(b) betweenthe thermally responsive member 90 and the adjacent trace segments 34and 36 should be sufficient to avoid any meaningful electricalconductivity between the trace segments 34 and 36 via the thermallyactuatable member 90, such as by arcing across the gap portions 80 a-b.In one embodiment, the two widths W_(a) and W_(b) are approximately thesame. Widths W of about 20-100 microns, e.g., about 50 microns, areexpected to be suitable.

The dimensions of the elements of the thermally actuatable switch can bevaried depending on the particular application. In one embodiment, thedistance between the conductive trace segments 34 and 36 (i.e., thewidth of the gap 80 in FIG. 4A) is between about 0.5 and about 2.0millimeters. The thickness (T in FIG. 4A) of the trace segments 34 and36 may be on the order of about 12-18 microns, e.g., about 12-15microns. The thermally responsive member 90 may have a thickness whichis the same as the thickness of the trace segments 34 and 36. It may bepossible to utilize a thermally responsive member 90 which is thickerthan the traces 34 and 36, but in one embodiment of the invention thethermally responsive member 90 is no thicker than the trace segments 34and 36. Prior to thermal actuation (i.e., as shown in FIGS. 4E-F), thethermally responsive member 90 in this embodiment of the invention has athickness no greater than the thickness T of the trace segments 34 and36, e.g., about 12-15 microns.

The trace segments 34 and 36 of the switch 50 shown in FIGS. 4 and 5 canbe formed of any suitable conductive material and may be arranged on thesubstrate in any appropriate fashion. The first trace segment 34 may beformed of a first conductive material and the second trace segment 36may be formed of a second conductive material. In one embodiment, thefirst and second conductive materials are different from one another. Inanother embodiment, the first and second conductive materials are thesame material. For example, the first and second conductive traces 34and 36 may be formed of copper or a copper alloy applied to the exteriorsurface 22 of the substrate 20. A number of techniques for forming suchtraces from a variety of conductive materials are well-known in the PCBmanufacturing arts.

The thermally responsive member 90 is formed of a thermally responsivematerial which is adapted to wet the exposed switch surface 82 whenselectively heated above an activation temperature. The activationtemperature may comprise a temperature at which the thermally responsivematerial becomes flowable so it can span the gap between the twoconductive trace elements 34 and 36.

FIG. 5 illustrates the thermally actuatable switch 50 after it has beenthermally actuated to close the switch 50. The thermally responsivemember 90 of FIGS. 4E-F has been heated above its activation temperatureand has flowed to span the entire width of the gap 80 between the tracesegments 34 and 36. This forms a conductive fused bridge 92 whichelectrically connects the two trace segments 34 and 36 to form a longerconductive trace.

In one embodiment, the thermally responsive material is a fusiblematerial and the thermally responsive member 90 may be referred to as afusible member 90. In one embodiment, the actuation temperaturecomprises a melting temperature of such a fusible material. In anotherembodiment, the activation temperature comprises a glass transitiontemperature T_(g) of the fusible material. The activation temperature isdesirably less than the melting point of the conductive material(s) ofwhich the conductive trace segments 34 and 36 are formed. This permitsthe thermally responsive member 90 to be heated sufficiently to flow andclose the switch 50, as shown in FIG. 5, without unduly damaging ormelting the conductive trace segments 34 and 36. In another embodiment,the activation temperature is no greater than a melting point of thesubstrate 20 or a temperature at which the thermally responsive materialwould react with the substrate 20. This temperature may be higher orlower than the melting point of the conductive materials used in thetrace segments 34 and 36. In one embodiment, the activation temperatureis no greater than 300° C.

In manufacturing some microelectronic device assemblies, microelectroniccomponents are mounted to a circuit board by reflowing an eutecticsolder. In embodiments of the invention employing eutectic solder tomount microelectronic components 70 to a circuit board 10, theactivation temperature of the thermally responsive member 90 may begreater than the melting point of the eutectic solder. This permits theentire microelectronic device assembly to be heated sufficiently tocause the eutectic solder to reflow and mount the components withoutactuating the switches. Some eutectic solders known in the art havemelting points of 220° C. or less. One embodiment of the invention,therefore, employs a thermally responsive material having an activationtemperature of at least about 220° C. In one particular embodiment, theactivation temperature is between about 220° C. and about 300° C. Inanother embodiment, the activation temperature is about 240-300° C.

Materials which are believed to be suitable include metals, metalalloys, and conductive organic materials. For example, metals and metalalloys having melting points between about 220° C. and about 300° C.include tin, high lead solders, high tin solders, lead-free solders, andother metal alloys. If so desired, the thermally responsive member 90may be coated with an organic solderability preservative (OSP), avariety of which are commercially available from a number of sources.

The material of the thermally responsive member 90 may sufficiently wetthe exposed surface 22 of the substrate 20 to readily wet the gapsurface portions 82 a-b and electrically connect the trace segments 34and 36. The wetability of this interface may be improved, however, byproviding a wetable coating 83 which is more readily wetted by thematerial of the thermally responsive member 90 as it flows. The wetablecoating 83 may, for example, comprise a metal. In one embodiment, thewetable coating 83 comprises a thin coating of gold. It is anticipatedthat a thin flash coating would suffice to improve wetability withoutcreating any unintended electrical connection between the trace segments34 and 36.

In the embodiment of FIGS. 4 and 5, the thermally responsive member 90is shown as being a single monolithic structure. In an alternativeembodiment shown in FIGS. 6 and 7, thermally actuatable member 100comprises two or more layers. In the illustrated embodiment, thethermally actuatable member 100 comprises a wetable base 102 and athermally responsive cap 104. The base may be formed of a material whichhas an activation temperature the same as or less than the activationtemperature of the fusible cap 104. In another embodiment of theinvention, however, the wetable base 102 will remain substantially solidand will not flow when the fusible cap 104 is heated to its activationtemperature. This permits the fusible cap 104 to flow to cover theexposed gap surface segments 82 a-b while leaving the wetable base 102substantially intact. FIG. 7 illustrates such a structure after thethermally responsive member 100 has been thermally actuated to form afused bridge 105 across the entire width of the gap 80.

In one embodiment, the wetable base 102 may comprise copper or gold andthe fusible cap 104 may comprise one of the materials noted above or thefusible material of the thermally responsive member 90 of FIGS. 4 and 5.In another embodiment (not shown), the fusible cap 104 may comprise twoor more layers. For example, the thermally responsive member 100 maycomprise a copper or gold base 102 carrying a first layer of one of thefusible materials noted above in connection with the thermallyresponsive member 90, and a second layer of gold over the first layer.The gold can help protect the underlying material of the fusible capfrom oxidation and the like, reducing or eliminating the need for anOSP.

In the embodiments of FIGS. 4-7, the trace segments 34 and 36 and thegap surface 82 are exposed, outer surfaces of the circuit board (10 inFIG. 1). FIGS. 8-9 illustrate another embodiment of the inventionwherein the circuit board comprises a laminate structure having asubstrate 20, a layer of selectively configurable circuitry (onlyconductive trace segments 34 and 36 being shown) and an upper layer 130.The upper layer 130 may comprise any suitable material. In oneembodiment, the substrate 20 and the upper layer 130 are formed of thesame material, but other laminate pairings known in the art could beused.

FIGS. 8A-B illustrate sequential stages in one process for manufacturingthe thermally actuatable switch 150 of this embodiment. As shown in FIG.8A, the upper layer 130 may be brought into contact with an upperservice of the trace segments 34 and 36 to generally enclose a gap 120.An orifice 132 may be defined in the upper layer 130. The orifice may beopen and pass entirely through the thickness of the upper layer 130,providing a passage between the exposed surface 134 and the contactsurface 136 of the upper layer 130. The orifice 132 can be sealed,however, so long as it is in communication with the gap 120.

The orifice 132 may be formed in the upper layer 130 before the upperlayer 130 is brought into contact with the connective trace segments 34and 36 as shown in FIG. 8A. In an alternative embodiment, though, theorifice 132 is formed after the upper layer 130 is in position. This maybe done, for example, by laser machining.

A portion of the contact surface 136 of the upper layer 130 is exposedto the gap 120, defining a wetable gap surface 122. As so desired, thisgap surface 122 may be provided with a wetable coating 123 substantiallythe same as the wetable coating 83 discussed above. As shown in FIG. 8B,the orifice 132 may carry a thermally responsive member 140 formed of athermally responsive material. In one embodiment, the thermallyresponsive member 140 substantially fills the orifice 132 and extendsbetween the exposed surface 134 and the contact surface 136 of the upperlayer 130. This thermally responsive member 140 may be positioned in theorifice 132 in any desired fashion, such as by known plating techniques.The thermally responsive material may be added to the orifice 132 afterthe upper member 130 is in position atop the trace segments 34 and 36,as illustrated in FIGS. 8A-B. In an alternative embodiment, thethermally responsive member 140 is created in the upper layer 130 beforethe upper layer 130 is assembled with the substrate 20 to create thethermally actuatable switch 150.

FIG. 9 illustrates the thermally actuatable switch 150 after it has beenthermally actuated. The thermally responsive member 140 of FIG. 8B hasflowed to wet the gap surface 122. This defines a fused bridge 142 whichspans the width of the gap 120 to electrically connect the tracesegments 34 and 36.

In the embodiment of FIGS. 4-7, the thermally responsive member 90 or100 is physically positioned within the gap 80 between the conductivetrace segments 34 and 36. This places the thermally responsive member incommunication with the gap so it may readily wet the gap surface 82. Theembodiment of FIGS. 8 and 9 illustrate that physically positioning thethermally actuatable member 140 within the gap 120 is not necessary,though. This thermally responsive member 140 is actually positionedoutside the gap 120. The thermally actuatable member 140 is still incommunication with the gap and with the exposed gap surface 122, though.Consequently, when it is heated above its activation temperature, it canflow to form a fused bridge 142, as shown in FIG. 9.

Processor-Based System

FIG. 10 illustrates a processor-based system in accordance with oneembodiment of the invention. The processor-based system 200 may comprisea conventional personal computer, a portable computing device, acellular telephone, or any other system which employs a processor. Theprocessor-based system 200 includes a processor 220 in communicationwith a system bus 210. One or more input/output devices 230 may be incommunication with the system bus 210. The system bus 210 is also incommunication with a selectively configured circuit board 10 inaccordance with an embodiment of the present invention. The selectivelyconfigured circuit board may comprise a DIMM generally as shown in FIGS.1-3. As shown in FIG. 2, for example, such a selectively configuredcircuit board may include a plurality of closed thermally actuatableswitches 50 a-p and a plurality of open thermally actuatable switches52. The selectively configured circuit board 10 may communicate with thesystem bus 210 via the board leads L₁₋₁₆ (FIGS. 1-3).

Methods of Manufacturing Microelectronic Device Assemblies

Embodiments of the present invention provide methods for manufacturingmicroelectronic device assemblies which include a microelectroniccomponent and a circuit board. In the following discussion, reference ismade to the embodiment shown in FIGS. 1-5. It should be understood thatthis is merely for ease of understanding, though, and that methods ofthe invention need not be limited to the particular structures shown inthese drawings.

In accordance with an embodiment of the invention, one or moremicroelectronic components 70 and a circuit board 10 are provided. Eachof a plurality of component contacts 72 on the microelectroniccomponent(s) 70 are connected to one of a plurality of board contacts 32carried by the circuit board 10. A first one of the normally openthermally actuatable switches 50 a-p or 52 a-p may be identified forselective closure. The first switch, e.g., switch 50 a, may be locallyheated to selectively close the switch 50 a. This defines an electricalpathway between the board contact 32 a and the board lead L₁.

The thermally actuatable switch 50 a may be selectively closed byheating the thermally responsive member 90 of the switch 50 a andcausing it to flow, as discussed above. The heating may be carried outin any suitable fashion. Desirably, though, the switches 50 and 52 areadapted to be individually closed in response to a localized thermalstimulus without necessitating closure of any other thermally actuatableswitch. For example, the thermally responsive member 90 may beselectively heated by a laser or other focused heat source to atemperature above its activation temperature. The thermally responsivemember 90 will then flow to wet the gap surface 82 to define anelectrically conductive path between the two conductive trace segments34 and 36.

Either thereafter or simultaneously, each of the other switches 50 or 52which have been identified for closure can be locally heated to yieldthe desired final circuitry 30.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1-36. (canceled)
 37. A method of manufacturing a microelectronic deviceassembly including a microelectronic component and a circuit boardcomprising: electrically coupling each of a plurality of componentcontacts of the microelectronic component to one of a plurality of boardcontacts carried by the circuit board, the circuit board carrying aplurality of configurable traces associated with the board contacts andeach of the configurable traces includes at least one normally openthermally actuatable switch; identifying a first normally open thermallyactuatable switch from the plurality of normally open thermallyactuatable switches; and locally heating the first switch to selectivelyclose the first switch to define an electrical pathway between at leastone of the board contacts and at least one of a plurality of board leadscarried by the circuit board.
 38. The method of claim 37 wherein thefirst switch includes a thermally responsive member and a gap betweentwo conductive lengths of one of the configurable traces, selectivelyclosing the first switch comprising heating the thermally responsivemember.
 39. The method of claim 37 wherein the first switch includes athermally responsive member and a gap between two conductive lengths ofone of the configurable traces, selectively closing the first switchcomprising heating the thermally responsive member sufficiently to causeit to flow and define an electrically conductive path between the twoconductive lengths.
 40. The method of claim 37 wherein the first switchincludes a thermally responsive member, a gap between two conductivelengths of one of the configurable traces, and an exposed gap surface,selectively closing the first switch comprising heating the thermallyresponsive member and causing it to wet the gap surface.
 41. The methodof claim 40 wherein the thermally responsive member is heated to atemperature below a melting point of the conductive lengths.
 42. Themethod of claim 37 further comprising identifying a second one of thenormally open thermally actuatable switches and locally heating thesecond switch to selectively close the second switch to define anelectrical pathway between at least one of the board contacts and atleast one of a plurality of board leads carried by the circuit board.